Viewfinder for camera

ABSTRACT

This invention provides a viewfinder which is mounted separately from a taking lens on a camera and wherein the occupying space of a parallax-correcting prism is minimized to arrange freely its optical design. And also, this invention provides a viewfinder decreasing the difference of the diopter and the fuzziness of the image between normal photographing mode and macro photographing mode. To achieve these objects, this viewfinder comprises a prism arranged in the optical path of said viewfinder to correct the parallax, and a driving mechanism to slide said prism in the direction normal to the axis of said lens system.

This application is a continuation of application Ser. No. 07/557,062,filed Jul. 25, 1990, U.S. Pat. No. 5,160,954.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a viewfinder mounted on a camera separatelyfrom a taking lens, and more specifically to a viewfinder having a prismfor correcting a parallax relative to the taking lens upon a macrophotographing mode.

2. Description of the Prior Art

Heretofore there has been a camera including a viewfinder separated froma taking lens such as a lens shutter camera or a still video camera. Theviewfinders of these conventional cameras have in principle parallax bythe difference in the direction of optical axis from that of the takinglens. This parallax can vary according to the distance from a camera toa photographic object, and is needed to decrease in actualphotographing.

Besides a normal photographing mode, some of the conventional camerashave a macro photographing mode for taking a short-distance object whichcan not be taken by the normal photographic mode.

Since a normal mode camera is preset to minimize the parallax for normalphotographing, it has high quantity of parallax in the short distancefor macro photographing.

In order to minimize this parallax upon macro photographing, some of theconventional cameras have a rotatable correcting prism by standingacross the optical path of the viewfinder and other cameras have amovable visual field frame in the viewfinder optical system.

FIG. 12 shows a schematic view for inserting a prism. This reversedGalilean zoom viewfinder has a first lens group 41, a second lens group42 movable for variable power, and an eyepiece group 43. Themagnification of the viewfinder is changeable according to that of ataking lens. The parallax is decreased by inserting the prism 44 betweenthe first lens group 41 and the second lens group 42 and thereby bendsits optical path to the taking lens side upon macro photographing.

FIG. 13 is a schematic view showing the moving of the visual field frameof a viewfinder. This zoom viewfinder has a first lens group 51, asecond lens group 52 movable for variable power, a Porro prism forerecting images, and an eyepiece 54. A visual field frame 55 is mountedin the vicinity of the image formation position of a photographicobject. The parallax upon macro photographing is corrected by moving thevisual field frame in the direction normal to the optical path andthereby an accurate visual field is gained.

Referring to FIG. 12, the prism on the optical axis is rotated in orderto insert into the optical path and to extract from the path, andrequires a comparatively wide space for its rotation at the same time.Therefore the lens-arrangement is restricted by this space.

In FIGS. 12 and 13, a high magnification of the viewfinder causes agreat difference between the diopter (i.e., the magnification power ofthe viewfinder) upon macro photographing to a short distance object, andthat upon normal photographing to a standard distance object. A range ofthe above difference often surpasses the adjustable range of the diopterof a photographer's eye. Since a standard diopter is preset generallywithin the available distance for normal photographing, a large changeof the diopter for macro photographing from a standard diopter makesfuzziness of an image in the viewfinder upon macro photographing.

When the distance to a photographic object is defined as U, measured inmeters, the standard object distance for setting a diopter is defined asU_(o) measured in meters, and the magnification of the visual field inthe viewfinder is defined as M, the diopter difference between oneobject located at U₀ and another object located at U will be as follows:

    ΔD=(U.sup.-1 -U.sub.0.sup.-1)M.sup.2

For example, when M=1.2, U₀ =3 m, and the standard diopter D=-1, areeach defined, the result is ΔD=0.96, and D=-1.96 in the macro mode inthe case of U=1.

Both a high magnification of the viewfinder and a large differencebetween the standard distance for adjustable diopter and the actualdistance requires much change from the standard diopter. Recently, zoomratio, in another word, a difference between the focal distances of ashort focal distance side and a long focal distance side, intends tobecome large. Therefore, the diopter upon macro photographing by longfocal distance to a short distance object largely changes from standarddiopter for normal photographing.

SUMMARY OF THE INVENTION

To resolve the foregoing problems, the first object of the presentinvention is to provide a viewfinder wherein the occupying space of aparallax-correcting prism in the lens group is small and accordingly theoptical design can be freely carried out.

The second object of the invention is to provide a viewfinder which candecrease the difference from the standard diopter and diminish thefuzziness of the image.

To achieve the first object, a viewfinder for a camera of this inventionis characterized to be constituted so that a parallax correcting prismis disposed to slide in the direction normal to the axis of the finderlens group for inserting into the optical path and extracting from it.

To achieve the second object, a viewfinder of this invention is furthercharacterized in that at least one surface of the prism is powerful andcurved. In this constitution, the image of the viewfinder upon macrophotographing can be cleared by setting the curvature of the prismsurface so as to diminish the difference between the diopters ofmacro-photographing and normal photographing.

Although the prism can be inserted in a optional position, the fuzzinessof the image through the finder lens group can be diminished byinserting the prism into the object side rather than into the viewfinderlens group because of changelessness of the axis of the viewfinder lens(i.e., the optical axis is thus not changed internally of the viewfinderwhen the prism is inserted in front of the object lens of theviewfinder).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective side view showing the whole constitution of acamera including a viewfinder according to the present invention;

FIG. 2 is a plan view showing an arrangement of the optical elements ofa viewfinder;

FIG. 3 is a side view of FIG. 2;

FIG. 4 is a schematic view of FIG. 2;

FIG. 5 is a plan view of a prism for a viewfinder in the embodiment;

FIG. 6 is a front view of a prism for a viewfinder in the embodiment;

FIG. 7 is a perspective side view of a viewfinder in the embodiment asviewed from an object side;

FIG. 8 is a perspective side view of a viewfinder in the embodiment asviewed from an eyepiece side;

FIG. 9 is a plan view showing a slide plate part in the embodiment;

FIG. 10 is a plan view showing a click plate part in the embodiment;

FIG. 11 is a plan view showing a prism driving mechanism in theembodiment;

FIG. 12 is a schematic view showing an arrangement of optical elementsof a viewfinder using the conventional prism;

FIG. 13 is a semitic view showing an arrangement of optical elements ofa viewfinder not using the conventional prism;

DETAILED DESCRIPTION OF THE EMBODIMENT

The embodiment of the present invention will be described hereinafterwith reference to the accompanying drawings.

FIGS. 1 through 11 show the embodiment adapting the present invention toa zoom viewfinder.

FIG. 1 shows a camera including a viewfinder in accordance with thepresent invention, and the numerals 1, 2, and 3 designate a camera body,a taking lens, and a viewfinder respectively. In the embodiment, theside mounting the taking lens 2 is defined as a front side 1a, and theside mounting the viewfinder through which a photographer gazes isdefined as a back side 1b. The broad surface of the upper part of thebody is defined as an upper surface, mounting a release button and azoom lever. The broad surface of the lower part of the body 1 is definedas a lower surface 1c, mounting a macro lever 4 for switching a cameramode between modes of macro and normal photographing. The macro lever 4is disposed on the position where the photographer can operate it withthe thumb F in having the camera.

The taking lens 2, of which focal distance can be changed by operating azoom lever, is a zoom lens. The lens 2 can be disposed or a suitableposition for macro photographing upon a macro photographing mode.

Referring to FIGS. 2 and 3, the optical system of the viewfinder 3 hasthe arrangement of the order from the object side of a first lens group10 having a negative power, a second lens group 11 having a positivepower as a whole of the adhered lenses, a third lens group 12 having apositive power, a mirror 13, a Porro prism 14 for erecting an image, andan eyepiece lens 15. The first and second lens groups 10 and 11 aredisposed to move in the direction of the optical axis Ax in order tochange the magnification of the viewfinder 3. And the visual field ofthe viewfinder can be changed by the lens groups' moving in thedirection of the axis according to the zooming of the taking lens 2.

FIG. 4 shows a schematic plan view of the viewfinder optics includingthe Porro prism 14. The 1st through 3rd lenses comprise an objectoptical system. A visual field glass 15, on which frames for the visualfield and for measuring the distance, etc., are each drawn, is disposedin the vicinity of an image-formation surface by the object opticalsystem.

A prism 20 is slidably disposed in the front of the first lens group 10in order to insert into or deflect from the optical axis of theviewfinder 3.

As shown in FIGS. 5 and 6, the prism 20 is formed in the shape of theletter L when seen in a plane view of the prism or in the shape of arectangle when seen in the front view of the prism. Portions of theprism inserted in the axis are a curved surface having a predeterminedpower in the object side surface 21, a plane predetermined power in theobject side surface 21, a plane slanting at a predetermined angle to thelens system axis Ax in the surface 22 of the first lens group 10 side,and an approximately wedged shape in a sectional surface of the slidingdirection of the prism 20. The wedge shaped section of the prism 20 canbest be visualized by viewing a cross-section as revealed by a sectionline extending horizontally through the FIG. 6 view of the prism. Thecurved surface 21 is a convex spherical surface having a large radius ofcurvature which is predetermined so that the difference between theobject diopters upon macro and normal photographing decreases. Thesurface of the curved surface 21 can be formed in not only a sphericalsurface but also a non-spherical surface in order to correct anaberration or the like.

A pair of guide pins 23, 24 projecting upward and downward are eachmounted on the upper and lower surfaces of the prism 20, and acam-follower pin 25 is also mounted on the lower surface of the edgeportion. The prism 20 is slidingly inserted in the front of the lenssystem and then bends the optical axis of the viewfinder to the axis ofthe taking lens side for correcting the parallax and controlling thedifference from the standard negative diopter by changing to a plusdiopter upon macro photographing.

The insertion of the prism 20 in the front of the lens system does nothave a bad effect on the function of the lens system because ofchangelessness of the optical axis in the lens system. Therefore theinferiority of the viewfinder image can be checked. On the other hand,the insertion in the middle of the lens system makes the inferiority ofthe viewfinder image because of the change of the axis between theinsertion and extraction.

The description on the mechanical constitution of a viewfinder will begiven in accordance with FIGS. 7 through 11.

FIG. 7 is a perspective view of the viewfinder as viewed from aphotographic object.

The viewfinder 3 has a casing 5 including the foregoing lens system inthe inside and comprising a frame unit 6 having a slidable first lensgroup 10, and a frame unit 7 having a slidable second lens group 9. Eachframe units 6 and 7 is constituted slidably in the sliding axes 6a and7a fixed to the casing. Each frame units is driven by a drivingmechanism (not shown ) connected with the stopping pins 6b, 7b accordingto zooming of the taking lens.

On the surface facing to the object of the casing 5, a pair of guiderails are mounted along the line from the optical axis of the viewfinderto the front side 1a of the camera body 1. These guide rails have guidegrooves 8a, 9a engaging guide pins 23, 24 of the prism 20. And the railsare disposed both upper side and lower side of the optical path of theviewfinder. A pair of the rails 8, 9 cross the optical path of theviewfinder. A straight part of these rails crosses the optical path, anda curved part of the rails is disposed out of this path. The prism 20can move slidably by the guide rails 8, 9 and thereby insert into orextract from the optical path of the viewfinder.

The boxes A, B fixed to the casing on the upper and lower positions onboth sides of the prism are a light-emitting unit and a light-receivingunit respectively to measure the distance to the photographic object, asshown in FIG. 7.

Hereinafter, the mechanism for driving the prism 20 will be described.Referring to FIG. 8, a macro lever 4 projecting from the lower surface1c of the body is fixed to a slide plate 30 mounted inside the body. Theprism 20 can slide by operating the macro lever 4 via a deliveringmechanism 31.

A slide plate 30 fastening the macro lever 4 is formed approximatelyrectangular-shaped and can go back and forth in the direction D1(direction vertical to the viewfinder axis) in which the photographer'sthumb F bends. As shown in FIG. 9, the slide plate 30 is constituted ofa engaging protuberance 30a for giving a click-like impression insliding by a click plate 32, and a engaging pin 30b for engaging withthe delivering mechanism. In FIG. 10, a elastic click plate 32 formed inthe T-shape is constituted of a lug hole 32a for the camera body, andengaging holes 32b, 32c for engaging with the protuberance 30a of theslide plate 30. The part of the click plate 32 facing to the slide plate30 can be movable in the upward or downward direction D2 in FIG. 8. Theengaging holes 32b, 32c have a function of determining a first andsecond stop positions.

In FIGS. 8, 11, the delivering mechanism 31 for delivering the motion ofthe slide plate 30 to the prism comprises an attaching shaft 32d fixedto the body 1 and a driving lever 33 movably attached to the attachingshaft 32d. The driving lever 33 comprises two fork-shaped arms 33a, 33bextending each other in the opposite direction. One arm 33a is engagedwith the engaging pin 30b of the slide plate 30 by the slot portionformed in its tip. And another 33b, bending its tip into a U-shape inorder to compensate the difference of the height between the prism 20and the slide plate 30, is engaged with a projecting cam follower pin 25of the prism 20 penetrating the guide rail 9 through a long and narrowhole 33d in the tip.

The function of a prism-sliding mechanism of the viewfinder according tothe embodiment will be hereinafter described on FIGS. 8 and 11.

The slide plate 30 is disposed on the position as shown by a dotted linein FIG. 11 by operating the macro lever upon normal photographing. Theengaging protuberance 30a of the slide plate 30 is engaged with theengaging hole 32b of the click plate 32, and the slide plate is disposedon the first stop position, and then the prism 20 is disposed on theposition outside the optical path of the viewfinder upon normalphotographing.

On the other hand, the slide plate 30 is moved by the macro lever to theposition of a phantom line (two-dot-and-chain line) in FIG. 11 uponmacro photographing. This moving makes the driving lever 33 pivot fromthe solid line position to the phantom line position in FIG. 11, andconsequently the prism 20 is slide into the optical axis of theviewfinder along the guide rail. When the prism 20 is disposedcompletely inside the optical axis, the engaging protuberance 30a of theslide plate is engaged with the engaging hole 32c of the click plate 32and the slide plate 30 itself is disposed on the second stop position.

In this embodiment, a photographing can be quickly operated by theswitching from the normal mode to the macro mode with the thumb F,because the macro lever 4 is mounted on the lower surface 1c of thecamera body 1.

What is claimed is:
 1. A viewfinder adapted to be mounted on a cameraseparate from a photographing lens of said camera, said viewfindercomprising:a lens system having an optical axis; a prism for correctingparallax between the photographing lens and said lens system and beingadapted to be selectively disposed along said lens system optical axis;said prism comprising a curved surface having a magnifying power forcorrected parallax and changing the diopter of said lens system when anobject is being photographed in close-up photography; and driving meansfor inserting said prism into said optical axis of said lens system uponclose-up photography and for extracting said prism from said opticalsystem of said lens system upon normal photography.
 2. The viewfinderaccording to claim 1, said lens system comprising a variable poweroptical system.
 3. The viewfinder according to claim 1, furthercomprising means, extending transversely to said optical axis forguiding said prism for slidable movement by said driving means.
 4. Theviewfinder according to claim 3, said guiding means comprising spacedguide rails positioned along the path of movement of said prism.
 5. Theviewfinder according to claim 3, said guiding means comprising railsextending transverse to said optical axis for guiding said prism.
 6. Theviewfinder according to claim 1, further comprising a close-upphotographing lever mounted for movement in a direction substantiallytraverse to said lens system optical axis.